1. Field of the Invention
The present invention relates to an induction motor control device and its control method for improving the accuracy of speed control.
2. Description of Related Art
The calculation of a slip angular frequency ωs of an induction motor is carried out according to Equation (1) using the inverse of a secondary time constant (=Lm/R2) of the motor:
                              ω          ⁢                                          ⁢          s                =                              R            ⁢                                                  ⁢                          2              ·              I                        ⁢                                                  ⁢            δ                                              Lm              ·              I                        ⁢                                                  ⁢            γ                                              (        1        )            where R2 denotes the secondary resistance, Lm denotes the mutual inductance, Iγ denotes the γ-axis current, and Iδ denotes the δ-axis current.
Since the secondary time constant of the motor includes the secondary resistance (R2), temperature fluctuations of the motor may cause an error in the calculated value of the slip angular frequency ωs. Thus, speed-sensorless speed control may cause speed accuracy deterioration. For example, Japanese Unexamined Patent Application Publication No. 7-75398 proposes a technique for improving the accuracy of calculating a slip angular frequency by detecting a terminal voltage and current of an induction motor, computing excitation energy stored in the excitation inductance of the induction motor, and adjusting the secondary time constant of the motor.